In a variety of contexts, it is desired to amplify an ac input signal by means of an operational amplifier, whereby any offset (dc component) in the input is removed by the amplifier while only the ac components in the input are amplified. For example, in a circuit for detecting the presence versus absence of an ac carrier signal wave of amplitude greater than a given threshold level in an incoming input wave signal, the incoming signal is fed serially through an operationl amplifier, a (full-wave) rectifier, and a low pass (smoothing) filter to a threshold detector. Typically, the threshold detector is a Schmitt trigger detector which can be trimmed by an auxiliary threshold trimmer circuit in order to adjust the threshold of signal to be detected. When the carrier wave is present in the incoming signal, the output of the amplifier contains an ac component which is an amplified voltage representative of the carrier. In response thereto, the rectifier delivers a rectified amplified voltage representative of the amplitude of the carrier to the low pass filter which in turn delivers an above-threshold dc voltage to the threshold detector. Thus, when the carrier wave is present in the incoming wave with an amplitude that is above the detector threshold, the presence of the carrier is detected as is desired. At the same time, however, the incoming signal can contain an unpredictable amount of signal offset (dc component), and any such offset in the incoming signal will also cause delivery to the threshold detector of an above-threshold dc voltage of unpredictable amount, whereby the detector falsely detects the presence of a (nonexistent) carrier in the incoming signal, because there is undesirably no essential difference in the voltage that is delivered to the detector in the case where the incoming signal contains dc offset without carrier and the case where the incoming signal contains carrier (with or without dc offset). This problem is excerbated in a typical situation where the incoming carrier wave amplitude is only about 7 to 10 millivolts while the offset in the incoming wave can attain a value as high as 100 millivolts of either polarity and saturate the amplifier, whereby amplification of the carrier is undesirably reduced to zero.
Another problem that occurs in amplifying the carrier arises from the inherent dc offset of the amplifier, typically about 5 millivolts of either polarity. Such amplifier offset typically is a steady but not precisely predictable value and could be compensated for by trimming the threshold detector, but this offset is also undesirably amplified, with gain substantially above unity, by the amplifier. Hence, this offset could not easily be compensated by trimming the threshold, because an ordinary trimmer circuit cannot handle such a large offset, i.e., an offset which is thus about as large as the amplitude of the rectified carrier.
It would therefore be desirable to have an amplifier circuit arrangement for amplifying an input ac signal whereby any offset that is present in the input ac signal is eliminated, and whereby the inherent dc input offset in the amplitude is not amplified with gain substantially above unity.